Length-measuring devices used, inter alia, in coordinate-measuring machines or machine tools conventionally employ a glass scale having a longitudinal succession of graduations that are scanned by a photoelectric sensor system. And recently, it has has become known that a second scale, alongside the measurement graduations, and in the form of several parallel lines extending in the lengthwise direction of the scale, can be an important component of a device for determining the degree of linearity of a given measurement of the graduations. In this connection, reference is made to U.S. Pat. No. 4,587,622 as well as to Federal Republic of Germany Patent Application 3,542,514.8.
The parallel lines of the scale of such a linearity-determining device serve to detect guidance errors of the measuring machine or machine tool, and therefore stringent requirements are imposed on the means of supporting the scale whereby to assure shape stability in the scale. And the type and manner of attaching the scale to the machine presents special problems through deformations of the scale due to transverse forces which result from any difference in thermal expansion of the scale and its holder or mounting.
It is already known that thermal stresses can be avoided by mounting a scale to its support by means of a layer of elastic adhesive, as is described in Federal Republic of Germany Patent Application OS 2,505,587. A further proposal has been to use, instead of the continuous elastic layer of adhesive, several pieces of rubber interrupted by intervening spaces; the scale is clamped on these pieces of rubber which are intended to compensate for the thermal stresses between the scale and the holder. Even with this type of attachment, which is described, for example, in Federal Republic of Germany OS 3,312,534 and in U.S. Pat. No. 4,569,137, residual forces still remain to an extent which intolerably deforms the scale. The principal disadvantage is that, even after temperature compensation, the scale no longer returns reproducibly to its initial position but instead, because of frictional forces, assumes an undefined new position. As a consequence of long-duration scale-constraining forces in the mounting, deformations have been observed that can be interpreted as flow of the glass of which the scale is made. All this may lead to errors in measurement, both upon position measurement in the lengthwise direction of the scale and, to a much greater extent, also in translation measurement values obtained from the linearity measurement device.